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Researchers using NASA's Far Ultraviolet Spectroscopic Explorer satellite have made the first ever direct observation of Eta Carinae's companion star.

Astronomers have good indirect evidence that a companion star exists. They have observed a five and a half year period repeating pattern of changes in the star's visual, X-ray, radio and infrared emissions.

Once every five and a half years, the X-rays from the region disappear completely.

Interestingly, Eta Carinae itself is too cool to produce X-rays, but it does have a stellar wind blasting particles into space at 300 miles per second. A collision between this wind and a similar one from a companion star could produce the observed X-rays.

This sparked suggestions that Eta Carinae does indeed have a companion, and that the X-rays drop off when the companion moves in front of the region where the two stellar winds collide, blocking it from Earth's view.

Eta Carinae is one of the most massive stars in the galaxy, according to NASA, and is thought to be nearing the end of its life. Its instability and odd quirks have made it one of the most studied stars of the last few decades.

Scientists have long suspected that it might have a companion star, because of its strange behaviour. But the research from the scientists led by Dr. Rosina Iping of the Catholic University of America in Washington, is the first direct evidence that the companion star is there.

Iping and her team used the Far Ultraviolet Spectroscopic Explorer satellite because it can detect shorter wavelengths than can Hubble. Hubble had previously scanned the region and found nothing.

The astronomers turned the Far Ultraviolet Spectroscopic Explorer satellite to look at the star just ahead of the regular cessation of X-rays from the region. When the X-rays stopped, there was also a noticeable drop off in far ultraviolet light, which Eta Carinae is also too cool to produce.

They concluded that when the X-ray emitting region is eclipsed, so too is the companion star, hence the drop off in far UV light.

Dr. George Sonneborn, Far UV project director at NASA's Goddard laboratory noted: "This far ultraviolet light comes directly from Eta Carinae's companion star, the first direct evidence that it exists. The companion star is much hotter than Eta Carinae, settling a long-standing mystery about this important star." ®